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H is a measure of the basicity of the nucleophile relative to protons, as defined by the equation: H = p K a + 1.74 {\displaystyle \ H=pK_{a}+1.74} where the pK a is that of the conjugate acid of the nucleophile and the constant 1.74 is the correction for the pK a of H 3 O + .
Pauling's second rule is that the value of the first pK a for acids of the formula XO m (OH) n depends primarily on the number of oxo groups m, and is approximately independent of the number of hydroxy groups n, and also of the central atom X. Approximate values of pK a are 8 for m = 0, 2 for m = 1, −3 for m = 2 and < −10 for m = 3. [28]
The Edwards equation relates the nucleophilic power to polarisability and basicity. The Marcus equation is an example of a quadratic free-energy relationship (QFER). [citation needed] IUPAC has suggested that this name should be replaced by linear Gibbs energy relation, but at present there is little sign of acceptance of this change. [1]
The higher the proton affinity, the stronger the base and the weaker the conjugate acid in the gas phase.The (reportedly) strongest known base is the ortho-diethynylbenzene dianion (E pa = 1843 kJ/mol), [3] followed by the methanide anion (E pa = 1743 kJ/mol) and the hydride ion (E pa = 1675 kJ/mol), [4] making methane the weakest proton acid [5] in the gas phase, followed by dihydrogen.
For metal oxides acidity and basicity are dependent on the charge and the radius of the metal ions as well as the character of the metal oxygen bond. The bond between oxygen and the metal is influenced by the coordination of the metal cations and the oxygen anions as well as the filling of the metal d-orbitals. [16]
The essence of Brønsted–Lowry theory is that an acid is only such in relation to a base, and vice versa. Water is amphoteric as it can act as an acid or as a base. In the image shown at the right one molecule of H 2 O acts as a base and gains H + to become H 3 O + while the other acts as an acid and loses H + to become OH − .
As the name suggests, a non-nucleophilic base is a sterically hindered organic base that is a poor nucleophile.Normal bases are also nucleophiles, but often chemists seek the proton-removing ability of a base without any other functions.
The term acidity function is also used for measurements made on basic systems, and the term basicity function is uncommon. Hammett-type acidity functions are defined in terms of a buffered medium containing a weak base B and its conjugate acid BH + :